Process of making fine ceramic powders from aqueous suspensions

Abstract

The new preparation process for making fine high specific surface ceramic powders suitable as catalysts or precursors for ceramics uses lanthanum (or other rare earth lanthanide) oxide as one of the precursors. The oxide is mixed with water to form a liquid slurry, whereby it is transformed to the hydroxide by reaction with water. The resulting hydroxide slurry, which can be milled to reduce the particle size and to speed up the reaction, is then combined, while stirring vigorously to assure homogenous mixing, with a solution of required amount of remaining metal nitrate precursors, for example strontium and cobalt nitrates. The reaction between lanthanum hydroxide and transition metal nitrates produces a colored (color depending on the transition metal) slurry consisting of metal hydroxides suspended in aqueous nitrate solution with pH>2. This perovskite precursor slurry is spray-frozen and freeze dried. The freeze-dried material is transformed to perovskite by slow controlled calcination at temperatures above 550° C. For example calcination 12 h at 585° C. and subsequently 4 h at 620° C. produces nearly phase pure perovskite having specific surface area of >10 m 2 /g, depending on the composition. The perovskite precursor slurry can alternatively be processed first by spray-drying (instead of spray-freezing/freeze-drying) and subsequent calcination.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 
     
       1. A method of making a high specific surface ceramic powder comprising the steps of: a) admixing in one or more steps a reactive oxide powder with one or more aqueous solutions which water component reacts with said oxide to form a slurry of hydroxide, at least one of said aqueous solutions further comprising a cation salt, itself comprising an alkaline earth salt and a metal salt, which mixture is allowed to react and results in forming a slurry of particles comprising hydroxides in a solution of cation salts, said slurry of particles having a pH between about 2 and about 8;   b) spray-freezing and freeze-drying, or spray-drying said slurry, whereby dried particles are obtained; and   c) calcining the dried particles.   
     
     
       2. A method according to claim 1, wherein after step a), said slurry is milled to speed up reacting and to produce finer particles. 
     
     
       3. A method according to claim 2, wherein after step b), the dried particles are milled before calcination. 
     
     
       4. A method according to claim 1, wherein after step b), the dried particles are milled before calcination. 
     
     
       5. A method according to claim 1, further comprising the step of forming a ceramic shape and sintering into a dense ceramic body. 
     
     
       6. A method according to claim 5, wherein said dense ceramic body is a high temperature superconductor. 
     
     
       7. A method according to claim 5, wherein said dense ceramic body is an oxygen permeable membrane. 
     
     
       8. A method according to claim 1, further comprising the step of shaping a porous monolith and sintering into a mechanically strong porous monolith. 
     
     
       9. A method according to claim 1, wherein said oxide is selected from the group consisting of lanthanide oxide, alkaline earth oxide, barium peroxide and mixtures thereof. 
     
     
       10. A method according to claim 9, wherein said metal salt comprises a transition metal salt or a precious metal salt, or both. 
     
     
       11. A method according to claim 10, wherein said salt is nitrate. 
     
     
       12. A method according to claim 1, wherein said metal salt comprises a transition metal salt or a precious metal salt, or both. 
     
     
       13. A method according to claim 12, wherein said salt is nitrate. 
     
     
       14. A method according to claim 1, wherein said powder has a perovskite structure comprising:   A.sub.x A'.sub.1-x B.sub.y B'.sub.1-y O.sub.3,     wherein:   A and A' are each independently selected from the group consisting of lanthanide, lanthanide-alkaline earth cation, lanthanide-lead, lanthanide-silver, lanthanide-alkaline metal and mixtures thereof, B and   B' are each independently selected from the group consisting of metal, transition metal, precious metal and mixtures thereof,   x is between about 0 to 1, and   y is between about 0 to 1.   
     
     
       15. A method according to claim 14, wherein the powder has a perovskite structure essentially consisting of La 0 .66 Sr 0 .34 Ni 0 .29 Co 0 .69 Mn 0 .02 O 3 . 
     
     
       16. A method according to claim 14, wherein the powder has a perovskite structure essentially consisting of La 0 .66 Sr 0 .34 Ni 0 .3 Co 0 .7 O 3 . 
     
     
       17. A method according to claim 14, wherein the powder has a perovskite structure essentially consisting of La 0 .66 Sr 0 .34 Ni 0 .32 Co 0 .63 Fe 0 .05 O 3 . 
     
     
       18. A method according to claim 14, wherein the powder has a perovskite structure essentially consisting of La 0 .09 Sr 0 .1 Cr 0 .95 Y 0 .05 O 3 . 
     
     
       19. A method according to claim 1, wherein the powder has a perovskite structure comprising:   AB.sub.2 X.sub.3 O.sub.n     wherein:   A is selected from the group consisting of lanthanum, yttrium, samarium, europium, gadolenium, dysprosium, holmium and mixtures thereof,   B is selected from the group consisting of barium, strontium-calcium, barium-strontium, barium-calcium and thallium,   X is selected from copper, platinum, silver, tin, lead and mixtures thereof, and   n is about 7-m, wherein m is 0-1.   
     
     
       20. A method according to claim 19, wherein the powder has a perovskite structure essentially consisting of Y 1  Ba 2  CU 3  O 7 . 
     
     
       21. A method according to claim 1, wherein said powder has a perovskite structure comprising:   Ln.sub.2-x A.sub.x B.sub.y B'.sub.1-y O.sub.4     wherein:   A is a divalent or monovalent cation,   B and B' are each independently selected from transition metals,   x is between about 0 to 1, and   y is between about 0 to 1.   
     
     
       22. A method according to claim 1, wherein said powder is a mixture of perovskites and other oxides.